The present invention relates to an apparatus capable of image capturing for recording a captured image as digital data, that is, to an electronic still camera.
In recent years, an electronic still camera (so-called the digital camera) for recording an image as digital data into a memory has become popular. Image data recorded in an electronic still camera are outputted to, for example, a built-in monitor of a camera, an external CRT, a printer or the like and subjected to appreciation.
The electronic still camera is provided with a view finder, a monitor, etc., as a means by which the user confirms the field of view of the image pickup lens in capturing an image. Particularly, as an electronic still camera having a built-in monitor, there has been known one provided with a preview function for outputting an optical signal coming from the image pickup lens before capturing the image to be recorded. According to this type of camera, the user is able to confirm the balance and construction of the image to be captured, or to focus the image while monitoring a preview image outputted to the monitor. However, though there is a margin in the field of view, a LCD type is often used as the monitor, and the resolution and the contrast of the image projected on it are low. Accordingly, there has conventionally been the problem that the confirmation of the image cannot be sufficiently achieved due to the influence of a screen flare when the electronic still camera employing the built-in LCD monitor is used, particularly in a bright outdoor environment.
Furthermore, in the case of the electronic still camera, the size of the CCD constituting an image plane on which the image coming from the image pickup lens is formed is normally small, and therefore, the aperture of an optical stop placed between the image pickup lens and the CCD is relatively narrow. This has resulted in a deep depth of field, and particularly in the bright outdoor environment, it has been difficult to confirm the focus by means of the built-in monitor.
As one that improves the resolution and the contrast of the image on the monitor, there has been known a device that changes the brightness of the back light according to, for example, the peripheral illuminance. However, according to this method, there is little effect on improving the focusing.
For the purposes of confirming the image to be captured, focusing the image and producing other effects, the preview image is required to be more rapidly processed and outputted to the built-in monitor.
In an electronic still camera, the capacity of the recording medium provided in the camera is normally constant, and therefore, the number of frames capable of capturing images is limited. Concerning this, there has been known an electronic still camera with which the user sets a capture image size representing an image resolution before capturing an image. According to this type of camera, setting a desired capture image size according to the situation can control the amount of data of each image. For example, when it is desired to save the number of image frames to be captured, it is possible to set the capture image size to a small size for the reduction in amount of image data, thereby allowing the securement of the number of image frames to be captured.
However, an image captured with the setting of a small capture image size has the quality of an image obtained by thinning out an image captured with the setting of a large capture image size since the image data of the small capture image size are outputted while being enlarged and interpolated when outputted in a specified output size to the built-in monitor, printer or the like.
That is, the quality of the image outputted in the specified output size mainly depends on the set capture image size. Therefore, an image captured with the setting of a small capture image size is outputted, the resulting image sometimes becomes aesthetically degraded in terms of image quality for appreciation use.
As a method for reducing the amount of data of each image in the electronic still camera, there is one method of setting the aforementioned capture image size to be small, and, conventionally, a method of reducing the amount of data by compressing the captured image data has often been used together with the above method. Among such electronic still cameras capable of executing data compression, there is one in which the user sets the compression rate of image data before capturing an image.
According to this type of camera, the user can set the desired data compression rate to control the amount of data of each image depending on the situation. When it is desired to store a larger number of frames into a recording medium having a certain capacity, or when it is desired to increase the data transfer speed in data processing, the user can set the compression rate high to execute compression so that the amount of image data is reduced.
In such an electronic still camera, the compression is executed so that the image data becomes smaller with the setting of a higher compression rate, and on the other hand, the image quality becomes more degraded. The conventional electronic still camera has generally been provided with a band correcting section for controlling the frequency characteristic of a signal for suppressing, for example, noises and a characteristic control section such as a gamma correcting section for controlling a gradation characteristic for the adjustment of the image contrast, with those control sections controlling the signal characteristics influencing the image quality.
However, in the conventional electronic still camera, such a characteristic control has not been executed according to the setting of the compression rate. When an image is captured at a high compression rate, the degradation in image quality based on a change in contents of the image quality due to data compression, cannot be avoided even in high-resolution image capturing for obtaining a fine image, and therefore, it has often been the case where the intended image forming cannot be achieved.
As an imaging device for an electronic still camera, normally a CCD having a specified number of pixels is used. In such an electronic still camera, as the number of CCD pixels increases, the resolution increases and an image having a better image quality can be obtained. However, the capacity of the storage medium provided in the camera is constant. Therefore, particularly in the electronic still camera, which has many CCD pixels and handles a high-resolution image, the amount of data of each image becomes large and the number of image frames to be captured decreases.
This problem is improved by setting the image conditions such as the capture image size, the image data compression rate and so on before capturing an image and reducing the amount of data of each image. In this case, the image quality becomes degraded due to the setting of a small capture image size or the setting of a high compression rate for image data as stated before.
When the image data compression rate is set high in a high-resolution electronic still camera having many CCD pixels, a significant influence is exerted on the image quality and much time is required for the compressing process, for which the image processing to be executed in the electronic still camera sometimes becomes retarded as a whole. Therefore, in this type of electronic still camera, there has been the problem that the image processing speed is reduced relative to the degradation in image quality and the number of image frames that can be captured per time is small.
Furthermore, in the electronic still camera that uses a CCD as an imaging device, the subject of the objective image to be captured is first recognized as an optical signal in the CCD, thereafter subjected to processing in various signal control sections and finally stored as digital data in a memory.
In capturing an image, mainly intended for photographic appreciation, a human being or a natural object is often selected as a subject, and therefore, a setting for processing an image signal obtained from the CCD as a halftone image having a gradation property has preparatorily been performed in each signal control section in the prior art electronic still camera. However, particularly when an electronic still camera having a CCD of a single plate (i.e., one CCD plate) is used for capturing the image of a text comprised of, for example, letters and numbers, the ruggedness of the edge portions of the letters is exaggerated by the influence of the process for emphasizing the image outline, and thin letters possibly become very obscure.
A device capable of changing the mode of recording an object according to the type of the object has been known. This device has, for example, a mode of recording an object as a natural color or monochrome halftone image provided with a gradation property and a mode of recording the object as a binary image having no gradation property as the modes of recording the object, and processing the image signal based on a recording mode selected according to the type of the object. According to this, the text as stated before is processed as a binary image, and therefore, a text image whose edge portions are comprised of relatively smooth letters and numbers can be obtained.
However, in the prior art apparatus, though the obscurity of letters of the text as stated before is improved, there is observed a tendency that the thin lines of the letters become dotted in processing the text as a binary image, and this has often caused a recognition failure. Therefore, even if the electronic still camera capable of changing the recording mode according to the type of the object is considered, when a text to be subjected to image capturing contains thin lines in the letters of the text, it has been difficult to faithfully reproduce the thin lines of the letters included in the text.